Paper highlights: Prior art and detector trouble

Hello everyone, and welcome to the remaining 90% of 2013 (as of February 4), surely you’ve made good use of the first 10!

I’ve been tasked with writing a review paper which made me come across several noteworthy papers. Two topics in particular surprised me: the history of instrumentation and the depth of detector corrections. In the hope that they may be of some interest to some of you, a few words on them might not be amiss.

What may come as a bit of a surprise, as it did to me, is that the history of SAXS instrumentation turns out to have been quite advanced right from the start. Current papers on instrumentation discuss the benefits of the technological advances from focusing optics, three-pinhole systems, crystal monochromators and so on, so here is a quote from O. E. A. Bolduan in 1949: “Investigators have attempted to solve the peculiar requirements of this field by use of longer wave- lengths to increase diffraction angles [3] crystals for monochromatization and collimation [4-7] high intensity x-ray tubes [8,9] or cameras with focusing by means of curves [sic] crystals [9,1o] or totally reflecting plates [11] to increase the level of useful radiation […]“, with Bolduan himself discussing three-slit collimation to reduce parasitic scattering from slit #2. Publications of SAXS instruments on home-built rotating anode generators appear already in the 1950’s. After reading a lot of papers discussing the finer details of those instruments leaves me wondering what major improvements we have made in the 60 years since that paper, and one of them is of course in the field of detectors.

Before the advent of more advanced 2D wire-detectors, imageplates and CCD-based detectors, one was pretty much stuck with using either Geiger counters (0D) or photographic film for 2D images. The photographic film, though, behaves rather nonlinearly vis-a-vis incident radiation amounts, necessitating serious corrections before it could be used. 2D wire detectors were an improvement, but only suitable for quite low countrates (<100 Hz/pixel) and show a rather large point-spread function. CCD-based detectors brought improvements there but their dynamic range is typically not much higher than 3 to 4 orders of magnitude. Imageplates brought improvements with up to 6 orders of magnitude of dynamic range, but necessitate laborious read-out and erasure procedures and thus are ill-suited for time-resolved experimentation. Recently, direct-detection detectors appear to solve the majority of the issues of all prior detectors, at a hefty price (conveniently, the price-tag of the Pilatus detector comes permanently written on its housing: a Pilatus 100k costs about 100k-euro, a 300k costs 300k-euro and so forth).

Nevertheless, each of these detectors produces intensity that is in dire need of corrections. Reading up on these, here are my recommended papers for getting more insight into some of these. Firstly, there is S. L. Barna with a good list of corrections, with some focus on distortions found in CCD’s. Furthermore, in results shown by V. Le Flanchec that even image plate data needs to be corrected for read-out positional inaccuracies as well as some other effects. F. Né also indicates their darkcurrent corrections need special treatment, in particular to start counting from the time of their last erasure. Other names to look for insights on this topic are, amongst others, P. Boesecke, T. Zemb, A. Rennie, P. Jemian, and many more, especially those in the CANSAS groups. Through their work, my list of corrections (of a month ago) has by now increased to a whopping 20(!) data corrections that could be considered applicable for small-angle scattering data. This space is too small to describe them all, but there will be space elsewhere later on this year, so stay tuned!

Lastly, in unrelated news: we applied the previously presented MC method to obtain radius distributions for isotropically oriented, rod-like precipitates in metals, the draft has been made publication-ready and can be found here. We are looking for a journal to publish this in at the moment.